In the oil and gas industry a production tank is located on a well site for receipt of well production fluids when not otherwise directly tied into a pipeline system. In another situation, production tanks are located at a facility site to temporarily hold production fluids either before or after processing. A tank truck is used to transport the production fluid between production tanks or from one production tank at a well site to another production tank at a facility.
Typical production fluid is oil, water, solids and gas. The tank is monitored by the field operation staff and the production fluid is removed as and when required. Tank level monitoring systems are commonly used to monitor and control the fluid levels in such tanks and vessels.
There is often also an accumulation of water in the tank in addition to oil. The lighter gravity oil floats on the water. Accordingly, for production inlets at fixed elevations, the water has to be drawn off first before accessing the oil stored thereabove. A water or waste truck is employed to first remove the water. Unfortunately, there is a risk that the truck overdraws the water and also takes some of the valuable oil. Accidentally drawn oil is lost to disposal with the waste water at significant cost.
Thus it is known to provide an intake valve to a discharge line employing a floating valve element which engages or disengages from a valve seat in response to specific gravity of the liquid contained in the tank. The floating valve element floats in water, to maintain an open intake, and sinks in the lighter oil, closing the intake to avoid drawing product oil. An example of such a valve is described in PCT Application Publication No. WO 2013/049937 to Reconeco and EP Patent No. 518,346 to Bywater. Both documents disclose a cage housing a floating valve element and a valve seat. The valve element floats in water and if the water is above the valve seat, floats above from the valve seat for permitting flow of water through the pipe end. In oil, the floating valve element sinks, engaging the valve seat and prevents the oil from flowing through the pipe end. The valve modulates between an oil and water interface. Such an intake valve is effective but has some shortcomings.
In one aspect, existing tank inlets, such as 11.4 and 8.9 cm pipe ends, are simply open ended and need retrofitting with a valve. To access the pipe end, personnel are required to enter the tank. The pipe end can be located over 6.1 meters below an access hatch, requiring full entry into the tank. As the tank is an enclosed space, housing hydrocarbons and possibly hazardous vapours and gas, regulations require a full cleaning operation before access. Further, the tank contents are typically heated, such as by using steam tracing, resulting in temperatures of 60-85° C. within the tank.
For example, several days can be required for preparing the tank for entry by personnel to retrofit the pipe end with the valve assembly. The preparation process involves draining of the tank, a desanding operation, steaming, erecting of scaffolding for a welder to access the pipe end for welding of the valve to the pipe end. The entire process can cost about $10,000 to $40,000. The expense is almost equivalent to the cost of lost production for two days at typical production rates of 20-30 m3/d. Further, when back in production, if the valve element becomes stuck to the valve seat, and cannot be dislodged with backflow manipulation, then the process must be repeated for tank access and repair.
The existing floating valve elements are permanently enclosed within an elongate wire cage. Typically, the valve elements have a specific gravity based on a standard oil gravity for control of oil-water interfaces in many situations. However, not all oil gravities are alike, possibly resulting in a valve element that is too light to close in a timely manner before loss of valuable production oil.
There is a demonstrated need for a more effective methodology for retrofitting of an intake valve to a pipe end and a more reliable valve for minimizing loss of oil with waste water.